News archive

January 2008

Tuesday, 22 January 2008

Adult male siskin on feeder

FINCHES, DARWIN AND EVOLUTION

Ask people what wildlife they associate with Darwin and most will say 'giant tortoises and Galapagos finches'. Certainly if you want a text book example to illustrate the principles of evolution and speciation you need look no further than the finches of the Galapagos archipelago, although it took some time for this 'penny to drop' in Darwin's mind. Within a very short time (geologically speaking) a prototype finch from mainland Ecuador not normally known for its wanderings managed to gain a foothold on the Galapagos, spreading to breed on most of the islands. Over time, geographical isolation, extremes of rainfall (and its consequences for the vegetation and available food sources) and altitude, favoured some physiological differences brought about by random mutation. Firstly, races and then species developed from this once homogeneous coloniser. There are now some fourteen species of 'Galapagos finch' recognised and some adopt elaborate behavioural traits such as the use of 'tools' to extract ants from tree holes or drinking (tortoise) blood in times of extreme drought in their 'struggle for survival'. The most obvious adaptations are bill and body size allowing different seed sources to be exploited. So exhaustively have these finches been studied that on some islands every finch has been ringed, their ancestry is well recorded and the fate of hybrids is followed up - more than can be said for the local human populations.

However, one does not have to go as far s the Galapagos to see these processes in action. One has only to get out into the local countryside to see the array of finches, buntings, and sparrows to realise that the process which Darwin described as 'descent with modification' is alive and well. Compare the huge bill of the hawfinch and the delicate bill of the linnet with the contorted bill of the crossbill - these differences have not persisted by accident, they serve a purpose. The streamlined appearance of the redpoll contrasts with the dumpy silhouette of the greenfinch and is explained by the presence of a crop in the latter species which allows it to collect seeds rapidly and digest at leisure; whereas those finches without a crop feed continuously and usually take smaller seeds. These differences separate the fringillid finches from the carduelid finches. However, appearances can be misleading; claims for the three 'species' of redpoll (lesser, mealy and arctic) and the two crossbill species (common and Scottish) on the grounds of appearance do not appear to be supported by DNA analysis. There seems to be as much genetic variation within species as between species - so what constitutes a species and where does it leave our only endemic bird, the Scottish crossbill?

The sudden arrival in winter of the snow bunting, twite and brambling reflect a lifestyle of migration evolved to maximise their chances of survival, exploiting areas rich in food during the summer while escaping the hostile climate in the winter by moving to lower altitudes or other countries. Carving out an ecological niche which does not involve competing with other like species is what survival is all about.

Sexual dimorphism is common in finches as is the richness of song and there is ample evidence to support the view that the more flashy the plumage and the more complex the song of the male the greater chance exists of his genes being passed on to future generations - this concept of sexual selection was considered a very powerful evolutionary force by Darwin. Mind you, this has been a mixed blessing for some finches as the practise of catching and caging wild birds to keep as 'pets' persisted until relatively recently, and led to the local persecution of species such as the linnet, goldfinch and greenfinch not to mention our local canary - the yellowhammer. Like Darwin's interest in 'ornamental pigeons', his grandfather Erasmus Darwin had observed the hybridisation which occurred in unrelated finch species, normally separated geographically, when kept in close proximity by aviculituralists. These resulting 'sports', both pigeons and finches show how the processes of evolution can be hijacked by mankind for its own gratification. One spin off from the wild bird trade, aided by global warming, is the possibility of waxbills and zebra finches joining parakeets in becoming part of our local avifauna - it is only a matter of time but do we want it?

A more questionable interference with natural ecosystems is the importation of a great variety of foreign seeds as wild bird food. One has only to see the alien mix which pops up in the lawn in spring under the bird table for this to be apparent. Undoubtedly this is attracting more finch species into our gardens adding to the appeal of bird-watching with the general public; however it might be drawing our attention and concern away from what is happening in the wider countryside. If Darwin could be brought back for a day he would be horrified not only by the march of urbanisation but by the lack of wildflower meadows, hedgerows and stubble fields in the winter. Also, most finches are forced to 'turn carnivore' and feed their chicks with invertebrates to get enough protein to ensure their rapid growth. These species which have multiple broods struggle to find enough insects to sustain second and third broods which starve in the nest due to widespread pesticide use - this is probably the main reason for the decline of 'the Cockney Sparrow'. It is man's interference more than any other factor that accounts for the decline of Darwin's local finches, in much the same way as the production of cereal monocultures has had the opposite effect in Africa, producing flocks of quelia finches of plague proportions.

In summary, in a temperate climate given a longer geological timescale and with a history of man's activity, the processes influencing the evolution of our finches are more complex and the lineages are more obscured by time, than with their Galapagos counterparts. However the process of evolution is universal. Bromley's heritage, both historical and environmental, contributing toward Darwin's Theory of Evolution, widely described as the 'greatest thought ever had by man' needs to be recognised and protected.

Bob Francis